Robo-Sub Goes Deeper Than Ever but Still No Sign of MH370

PERTH, Australia -- A robotic submarine headed back down into the depths of the Indian Ocean on Friday to scour the sea floor for any trace of the missing Malaysian jet one month after the search began off Australia's west coast.

It was the fifth attempt by the Bluefin-21 unmanned sub to find wreckage or the black boxes from Flight 370 in a distant patch of seabed. Data from the sub's previous missions has turned up no evidence of the plane.

The sub, which can create sonar maps of the ocean bottom, has now covered 42 square miles of the silt-covered seabed, the search coordination center said. The sub's last mission hit a record depth beyond its recommended diving parameters, which can potentially cause risk to the equipment, the U.S. Seventh Fleet said in a statement. However, it is being closely monitored.

Officials are desperate to find some physical evidence that they are searching in the right spot for the Boeing 777, which vanished March 8 with 239 on board on a flight from Kuala Lumpur, Malaysia, to Beijing.

The Bluefin is searching a remote stretch of ocean floor about 15,000 feet deep in an area where sound-locating equipment picked up a series of underwater sounds consistent with an airplane's black box, but it went down to 15,404 feet during mission four.

Australian Prime Minister Tony Abbott has said officials are "very confident" the sounds came from the Malaysian jet's cockpit voice and flight data recorders, but finding the devices in such deep water is an incredibly difficult task.

Radar and satellite data show the plane flew far off-course and would have run out of fuel in a remote section of the Indian Ocean.